Power converters/inverters are commonly used in a machine for motor control. Power converters/inverters usually include a plurality of power transistors, and these power transistors may be switched on and off to modulate an output voltage from the power converter/inverter. Knowing the state of the current or voltage of each phase of the motor allows for a power converter/inverter to be controlled to produce a controlled power waveform to the motor. Sensors are often deployed to detect the current associated with each phase of the motor. If one or more sensors are producing incorrect current values, the commands and changes in voltage to control the AC motor will not match the actual state of the AC motor, potentially leading to a loss of motor control.
Because accurate detection of phase current is critical to ensuring synchronous operation of the motor, precision sensors are typically provided for each phase of the motor. Some conventional sensor control strategies provide back-up phase current detection schemes, should one or more sensors fail or otherwise become inaccurate or unreliable. Many such sensor control strategies often include a method to detect the failure of one or more of the sensors. A conventional control strategy may first detect if a sensor has failed by summing the detected current for each phase of the AC motor. If the sum is not zero, each detected current may be compared against an estimate value for the current of that phase of the AC motor. A sensor has failed when the detected current for that sensor does not match the estimated current of that sensor. If a current sensor has failed, and an alternate way to control the inputs to an AC motor is not available, the motor may have to be shut down. Therefore, in order to ensure proper operation of the motor in the event of a sensor failure, a relatively simple, inexpensive motor control strategy that can detect a sensor failure and compensate for such failure may be desirable.
One device and method for failure detection in electric vehicles is described in U.S. Pat. No. 5,357,181 to Mutoh et al. (“the '181 patent”). The '181 patent describes a device or method that may detect if one or more current sensors are failing, and provide an alternate way to control the motor to compensate for the failed sensor(s). The system of the '181 patent uses a predetermined maximum current error value, which defines a maximum total current level permitted on the system. The sum of the three phase currents is then compared to this error value to monitor the operating condition of the current sensors. If the summation is below this value, the sensors are in normal condition. If the summation exceeds the error value, the disorder in output from excessive input deviation of current control (the estimate and the sensed signal) is checked at the current control system of each phase. The current sensor of the phase at which a disorder is found is determined to be failing. If only one out of three current sensors is failing, the device or method estimates the current of the failing current sensor by using the two normal current sensors. If two or more current sensors are found to be failing, the motor is controlled based on the AC current reference.
Although the device and method of the '181 patent may provide a method of determining if a current sensor has failed and estimating a value for the failed current sensor, it may include several disadvantages. Specifically, the device and method of the '181 patent may require a large number of components, rendering the system unnecessarily costly and unduly complex. Additionally, because the device and method of the '181 patent maintain an estimate of the current of each phase to aid in the detection of disorder in output from excessive input deviation, small errors in one or more current sensors may cause the estimate of one or more current sensors to drift and, in the event of a current sensor failure, the wrong sensor or sensors may be determined to have failed. Furthermore, the device and method of the '181 patent may not re-qualify a current sensor that has been determined to have failed if that current sensor later begins to perform normally. Thus, in order to facilitate accurate and reliable motor control, a motor control sensor strategy that can detect and accurately correct for a failed sensor, while reducing the cost and complexity of the AC motor control sensor strategy by decreasing the amount of processing and the number of sensors required, is desirable.
The disclosed system and method are directed to improvements in the existing technology.